We asked folks on Twitter. Here are some of the (sensible) theories that came back:

There's still no power in many areas, so everyone needs gas to run their generators (DEMAND)

There's still no power in many areas, and gas stations can't pump without power. This has increased the load on the stations that do have power and sucked their tanks dry. (SUPPLY)

There's limited public transportation, so everyone has to drive (DEMAND)

Everyone's panicked that they'll run out of gas (DEMAND)

Refiners can't refine and deliver gas without power (SUPPLY)

On this last point, we heard from a former oil analyst who added some details:

It requires electricity to run a refinery, open ports to offload barges, and electricity to pump gas. Major major infrastructure issue. Generators add to demand but they don't use up high-test. And yes you can't run a gas station without power.